Antennas are generally characterized by their bandwidth properties into two categories—instantaneous bandwidth (IBW) and tunable bandwidth. Instantaneous bandwidth refers to the band of frequencies over which an antenna can maintain its (radiated) main beam in a fixed position in space. Tunable bandwidth refers to the band of frequencies over which an antenna exhibits well-matched input impedance at its input port.
In general, an antenna's instantaneous bandwidth is not equal to its tunable bandwidth. Many of today's SATCOM and Terrestrial point-to-point (PTP) communication systems require operation over larger instantaneous bandwidths (e.g., advanced extremely high frequency (AEHF), 1 GHz on receive and 2 GHz on transmit). Similarly, many radar systems (Synthetic Aperture Radar for example) require large instantaneous bandwidths for enhanced high-resolution imaging.
To achieve adequate signal levels over wide IBWs, the terminal antenna must maintain an uninterrupted connection over the entire bandwidth. This requires the terminal antenna main beam to remain trained on the axis of the satellite (or axis of the target) with minimal movement over frequency. For mobile terrestrial or aeronautical applications an additional requirement is that the antenna main beam remain trained on the satellite over a near-hemispherical scan volume (i.e., 10 to 90 degrees elevation, 0 to 360 degrees azimuth). In addition, to minimize aerodynamic drag, the antenna should be low profile.
Achieving both the aforementioned large IBW and near-hemispherical scan coverage in a low profile package can be challenging for traditional phase arrays due to the various hardware modules that are required (e.g., phase shifters and Variable True Time Delay (VTTD) components). Additional drawbacks to traditional phased arrays may include reduced ohmic efficiency, increased weight, and unacceptable height profile. These deficiencies may make a fully functioning traditional phased-array antenna cost prohibitive.
Some traditional antenna systems can achieve the desired IBW performance but do so usually at the cost of increased size, increased weight, and/or increased profile. Gimbaled reflector and slotted array antenna systems, for example, can be made to track a satellite over frequency and scan but usually require a high profile installation not compatible with most aeronautical and some terrestrial applications, particularly when low drag, low observable installations are required.
Phased arrays seem ideally suited for low profile installations. Along with being able to achieve a desired scan volume the phased array must be capable of maintaining a fixed or quasi-fixed beam position over the desired transmit or receive IBW at arbitrary scan. This may pose a problem for traditional phased arrays that are comprised of multiple radiating elements (or modules) fed with a passive corporate feed network having equal line lengths (true-time-delay) to all elements. In this case, beamwalk will be minimized only at broadside (i.e., a scan angle of 0 degrees). If instead the equal line lengths are adjusted to favor a scan angle other than broadside (e.g., a beam position of 45° at an azimuth of 0°), then the beam-walk will be severely degraded when the main beam is commanded to the diametrically symmetric beam position (i.e., a beam position of 45° at an azimuth of 180°), which risks potential loss of connection with a satellite and thus limits the usable scan volume of the array. This problem may be overcome somewhat by adding variable true time delay (VTTD) networks between the corporate feed and each radiating element of the array. Each VTTD allows the time-delay to be adjusted for scan angle (via a switchable N-bit “line-stretching” device). However, the addition of VTTDs (and discrete VTTD control devices) to the array increases the complexity, power consumption, weight and height profile of the overall system, all of which may be cost prohibitive.
The above deficiencies may be somewhat mitigated by modularizing the array into a set of discrete subarrays. Within each subarray a separate feed network distributes power to each individual element. The phase of each element within a subarray can be independently adjusted to scan the subarray (element factor) to a desired scan angle. Though a VTTD is still required between each subarray and the corporate feed, the total number of VTTDs will be less when using subarrays. Since the aperture area of each subarray is a fraction of the total array area, its 3 dB beamwidth will be many times that of the full array. While the main beam of each subarray will move with frequency, the total pattern as determined by the product of the subarray antenna pattern (element factor) and the corporate feed plus VTTDs (array factor) will move negligibly. This arrangement serves to provide good IBW while reducing the required number of VTTDs. However, the desired number of subarrays and VTTDs must be traded with the quantization side lobe levels (which may be excessive) and attendant directivity loss that will now be part of the full antenna pattern.